1. Field of the Invention
The present invention relates generally to transmission power control on a downlink shared channel (DSCH) used in an asynchronous W-CDMA (Wideband Code Division Multiple Access) mobile communication system, and in particular, to an apparatus and method for controlling transmission power by transmitting a TFCI (Transmit Format Combination Indictor) in a period of a downlink dedicated physical channel (DL-DPCH) assigned to a UE (User Equipment) which uses the DSCH in a handover (or handoff) zone.
2. Description of the Related Art
A downlink shared channel (DSCH) used in a European W-CDMA mobile communication system, a 3rd generation mobile communication system, is shared by a plurality of UEs. The DSCH is assigned to the UEs on a time division basis to transmit packet data or other high-speed data to the UEs in a 10 ms-radio frame unit. The DSCH can vary a data rate of transmission data in a frame unit, and can be subject to power control in a slot unit, like a dedicated channel (DCH) established between a Node B and a UE in the W-CDMA system. The radio frame, a basic unit for transmitting signals in the W-CDMA system, has a length of 10 ms, and each radio frame is comprised of 15 slots. In addition, the DSCH is a channel for transmitting user data only. Transmission power of the DSCH is controlled according to a TPC (Transmit Power Control) bit transmitted over an uplink dedicated channel (UL-DCH) assigned to the UE at the same time when the DSCH is assigned to the UE. Further, the TPC is also used for power control on the DL-DCH (Downlink Dedicated Channel) assigned to the UE in association with the DSCH (see 3GPP TS 25.214). The transport channel DL-DCH is transmitted over the physical channel DL-DPCH. The DSCH can be continuously transmitted to one UE over a plurality of frames. Alternatively, the DSCH can be transmitted to the UE over only one frame. The time to transmit the DSCH to a plurality of the UEs is determined by scheduling in an upper layer. The determined time information is provided to the UEs through a signaling message from the upper layer or through a TFCI bit for the DL-DPCH established in association with the DSCH.
FIG. 1A illustrates a structure of the DSCH. Referring to FIG. 1A, reference numeral 101 depicts a 10 ms radio frame of the DSCH, and reference numeral 103 depicts a slot Slot#i in the radio frame 101. As illustrated in FIG. 1A, the DSCH radio frame 101 is comprised of 15 slots Slot#0–Slot#14, and the DSCH slot 103 has a length of 2560 chips. An amount of information transmitted over the DSCH slot 103 is in inverse proportion to a spreading factor (SF) used for the DSCH slots, and the SF has a value of 4 to 256.
FIG. 1B illustrates a structure of the DL-DPCH for transmitting the DL-DCH assigned to the UE by the Node B. The DL-DPCH is assigned to the UE in association with the DSCH of FIG. 1A for power control on the DSCH and signaling even when the DSCH is in use. In FIG. 1B, reference numeral 111 indicates a 10 ms radio frame of the DL-DPCH, and each slot of the DL-DPCH radio frame is comprised of DATA1 113, TPC 112, TFCI 114, DATA2 115, and Pilot 116. Each slot of the DL-DPCH can have various structures according to the lengths of the DATA1, TPC, TFCI, DATA2, and Pilot.
The DATA1 113 and the DATA2 115 constitute a downlink dedicated physical data channel (DL-DPDCH), and the DL-DPDCH transmits user data and signaling information from the upper layer. The TPC 112, the TFCI 114, and the Pilot 116 constitute a downlink dedicated physical control channel (DL-DPCCH). The TPC 112 is a field for transmitting a command for controlling transmission power of uplink channels transmitted from the UE to the Node B, and the Pilot 116 is a field for enabling the UE to measure transmission power of a downlink signal for power control on the downlink signal. Further, the TFCI 114 is a field for transmitting a codeword indicating that transport channels having different data rates are transmitted over the DL-DPCH. The transmitted TFCI corresponds to one of 1024 TFCs (Transport Format Combinations).
The W-CDMA system uses a (32,10) coding scheme in order to increase reliability of transmitting the TFCI. The 1024 TFCs are represented by a 10-bit binary sequence, and the 10-bit binary sequence is coded into 32 coded symbols by a (32,10) coder. Among the 32 coded symbols, 2 coded symbols are punctured, and then, each slot transmits 2 coded symbols to the UE in a frame unit. That is, since each radio frame is comprised of 15 slots, it can transmit a total of 30 bits. Therefore, the 32 coded symbols are transmitted after 2 coded symbols are punctured. In addition, when the DL-DPCH is assigned in association with the DSCH, a TFCI for the DSCH and a TFCI for the DL-DPCH are transmitted simultaneously.
There are two methods for transmitting the TFCI 114 by separating it into the TFCI for the DSCH and the TFCI for the DPCH. As for the TFCI 114, 30 coded symbols are transmitted for one frame as stated above, and the 30 coded symbols constitute one TFCI codeword. Therefore, the TFCI 114 comprised of the 30 coded symbols should be divided into two TFCIs. A first method is called “logical split mode method” for logically separating (splitting) the 30 coded symbols instead of previously separating the TFCI into the TFCI for the DSCH and the TFCI for the DL-DPCH. A second method is called a “hard split mode method” for transmitting the 30 coded symbols by separating them into the TFCI for the DSCH and the TFCI for the DPCH. A detailed description of the two methods will be made herein below.
In the logical split mode, after decoding a 10-symbol TFCI codeword from the received 30 coded symbols, the UE analyzes some of the 10 decoded coded symbols for the DL-DPCH and analyzes the other coded symbols for the DSCH. In the hard split mode, some of the 30 coded symbols are transmitted as a TFCI for the DL-DPCH and the other coded symbols are transmitted as a TFCI for the DSCH, and then, the TFCIs are subject to separate decoding processes.
FIG. 2 illustrates downlink and uplink signal flows for the case where a UE receiving a DSCH is located in a soft handover zone, wherein for simplicity, only two Node Bs are considered. It is assumed herein that the respective Node Bs belong to different RNCs (Radio Network Controllers). The Node B and the RNC, terms used in the 3rd generation W-CDMA mobile communication standard, are elements of a UTRAN (UMTS (Universal Mobile Telecommunication System) Terrestrial Radio Access Network). The term “UTRAN” refers to all of the elements in the W-CDMA standard excepting the UE. The term “Node B” refers to a base station, and the term “RNC” refers to an element of the UTRAN, for controlling the Node B.
A soft handover (SHO) occurs due to mobility of a UE 211. When the UE 211 moves away from a current Node B in communication with the UE 211, and at the same time moves to an area where it can receive signals from an adjacent new Node B, the UE receives the signals not only from the current Node B but also from the new Node B. This state is called a handover state. In this state, if a quality (or level) of the signal received from the current Node B is less than a predetermined threshold, the UE releases the channel established to the current Node B, and then establishes a new channel to the new Node B providing high-quality signals, thus performing the handover process. By doing so, it is possible to maintain a call without interruption.
If the UE 211 arrives at a soft handover zone, the current Node B in communication with the UE 211 decreases its transmission power. This is to provide a smooth handover between the UE 211 and the current Node B. The UE 211 then performs simplex or weighted summation on the transmission power levels of the current Node B and the new Node B. Thereafter, the UE 211 requests both of the Node Bs to control their transmission power levels proper for the summed value. By doing so, it is possible to decrease not only a transmission power level of a signal transmitted from the Node B to the UE 211 in an active region but also a transmission power level of a signal transmitted from the UE 211 to the Node B in the active region, contributing to a decrease in interference between adjacent UEs and between adjacent Node Bs.
Referring to FIG. 2, a Node B1 201 serves as a primary Node B transmitting the DSCH and its associated DL-DCH to the UE 211, while a Node B2 203 serves as a secondary Node B transmitting only the DL-DCH to the UE 211 due to movement of the UE 211. A set of the Node Bs set to transmit signals to the UE 211 existing in an SHO zone is called an “active set”. When the UE 211 receiving the DSCH exists in the SHO zone, a problem occurs when the UE 211 receives the DSCH and the DL-DCH from the Node B1 201, but only receives the DL-DCH from the Node B2 203.
Here, the typical reason that the DSCH does not support the soft handover is because compared with the DL-DCH, the DSCH transmits data at a relatively high data rate, thus consuming an increased number of channel resources of the Node B. As a result, system capacity is affected. In order to enable the DSCH to support the soft handover, all of the Node Bs in the active set should have an algorithm for supporting the DSCH. However, to realize the algorithm, the Node Bs must be synchronized with one another. In addition, the W-CDMA mobile communication system may have a timing problem due to non-synchronization between the Node Bs. In order to support the SHO, the DSCH shared by a plurality of the UEs requires elaborate scheduling for the time points where it is used by the respective UEs. In light of the scheduling, it is difficult to embody transmission of the DSCH from the new Node B to the UE.
The DL-DCHs transmitted from the Node B1 201 and the Node B2 203 are received at the UE 211, and then subjected to soft combining. Here, “soft combining” refers to combining the signals received at the UE through different paths. The soft combining is aimed at reducing the interference, which affects the signals received at the UE 211, by summing the same information received through the different paths and then analyzing the summed value.
The soft combining is available only when the UE 211 receives the same information from the different Node Bs. However, when the UE 211 receives different information from the Node Bs, the received information, though subjected to soft combining, will be recognized as a noise component, resulting in an increase in the noise component of the signal. In the process of analyzing the DL-DCH, the downlink signals transmitted to the UE 211 from the respective Node Bs, i.e., the Node B1 201 and the Node B2 203, are subjected to soft combining except for the TPC bits 112 shown in FIG. 1B. The reason that the TPC 112 is analyzed separately rather than being analyzed by soft combining is because the TPCs received at the UE 211 from the respective Node Bs may be different from each other, since the signal received at the Node B1 201 from the UE 211 is high in level while the signal received at the Node B2 203 from the UE 211 is low in level, or vice versa, due to movement of the UE 211. Therefore, the TPC 112 is analyzed through a separate TPC analysis algorithm for a plurality of the Node Bs, rather than being subjected to soft combining.
As stated above, since the other fields of the DL-DCH except for the TPC field 112 are subject to soft combining, even the TFCI parts in the DL-DCHs transmitted from the Node B1 201 and the Node B2 203 are also subject to soft combining. That is, since the Node B1 201 transmits both the DL-DCH and the DSCH to the UE 211, it also transmits the TFCI for the DL-DCH and the TFCI for the DSCH.
The TFCI transmission method, as stated above, is divided into the logical split mode method and a hard split mode method. In the logical split mode, the UE 211 first decodes 30 TFCI coded bits received, and then separately uses the TFCI for the DL-DCH and the TFCI for the DSCH. Therefore, according to the W-CDMA standard, even though the Node B1 201 and the Node B2 203 belong to different RNCs, the Node B1 201 and the Node B2 203 can transmit the same TFCI coded bits. However, when the W-CDMA system transmits the TFCI for the DSCH in the hard split mode, no specification has been defined on a signaling method for transmitting the TFCI value for the DSCH to a Node B belong to another RNC. Therefore, when the Node B1 201 and the Node B2 203 in the active set of the UE 211 belong to different RNCs, the Node B2 203 does not recognize the TFCI value of the DSCH.
As stated above, since the UE 211 subjects the TFCI to soft combining after reception, the TFCI for the DSCH received at the UE 211 includes only the TFCI from the Node B1 201, unless the Node B2 203 transmits the TFCI for the DSCH. Therefore, the DL-DCHs received at the UE 211 from the Node B1 201 and the Node Be 203 are subject to soft combining, and then power controlled by the UE 211 considering a soft combined value. In contrast, as to the TFCI for the DSCH, the UE 211 considers only the Node B1 201, so that the UE 211 receives signals at unstable power. In this case, the TFCI for the DSCH may not be correctly analyzed.